There's no such thing as a "cubic gallon". A 'gallon' is already a unit of volume,
only without the word 'cubic' in it.
If you like to play with different units of measure, here are a few conversions:
1 gallon = 231 cubic inches
1 cubic foot = 1,728 cubic inches = about 7.4805 gallons
1 cubic meter = 1,000 liters = 1 million cubic centimeters = about 264.17 gallons